CELL-CONTENTS AND FORMS OF CELLS. 217 



processes of some of the lower plants, as Vaucheria, Diatoms, etc., 

 fixed oils rather than starch are formed in the chromatophores. 

 It is well known that in the cells of the bark of a number of plants 

 fixed oils are stored in place of carbohydrates. These facts show 

 that there is a very intimate relationship between the fixed oils and 

 other metabolic substances. Fixed oils constitute the reserve 

 materials in seeds, spores, pollen grains, and are even present in 

 the tubers of certain plants as Cyperus esculentus. The storing 

 of fixed oils instead of starch may be of some advantage to plants, 

 in that there is a greater supply of energy contained in them than is 

 present in the same quantity of any of the carbohydrates. Again, 

 as the specific gravity of the fixed oils is less than that of the carbo- 

 hydrates, this is an advantage in those spores or seeds which are 

 disseminated by the wind and require to be as light as possible. 



The fixed oils are more or less intimately associated with the 

 protoplasm occurring in vacuoles of the same in fruits and seeds. 

 The waxes which are secreted in the epidermal cells of leaves and 

 green stems, and also found as a covering of many fruits, serve to 

 protect the underlying cells from loss or excess of moisture, 

 from the attack of disease-producing micro-organisms, and also 

 prevent the interactions caused by some of their enzymes. The 

 resistance of certain micro-organisms, as the tubercle-bacilli, is 

 supposed to be due to some extent to the fatty substances in which 

 their bodies are enclosed or with which they are impregnated. 

 " It is held by some that the fats, or, more correctly, the lecithin 

 and phospholipines, are essential to the cohesion and physical 

 constitution of the protoplasm, so that any interference with the 

 physical state of these substances arrests the vital functions. The 

 cement which binds the organized matter together is loosened by 

 the solution in it of foreign substances, and it is the loosening 

 of the protoplasmic cement that makes it possible for the normal 

 processes of life to be carried on. 



" Attempts to form a concrete conception of the physical rela- 

 tionship in the structural organization of cells between fats on the 

 one hand and the other constituents of living matter on the other 

 have not been successful. Some have spoken of ' lipoid mem- 

 branes ' as if the living cell itself were enclosed in a fatty envelope 

 and accessible only to such substances as can permeate this envelope 



